1. Field of the Invention
The present invention relates to a method for manufacturing a piston employed, for example, in a compressor.
2. Description of the Prior Art
Pistons, which are usually made of metal such as gray cast iron or steel, has been improved in abrasion resistance of the outer surface, because it reciprocates very fast in cylinders. Japanese Patent Publication No. 41244/1979, for example, discloses that a piston made of aluminum has its outer surface processed into hard almite to enhance abrasion resistance. The outer surface of the piston is further hardened by cementation to enhance abrasion resistance more.
Meanwhile, pistons usually have a cavity inside and a thin and cylindrical skirt at least.
According to the foregoing, since the outer surface of the piston is processed by heat hardening such as cementation hardening and the skirt of the piston is thin and cylindrical, the skirt tends to flare or spread outwards to the bottom when heated. Therefore, polishing the outer surface of the piston as a finish, a part of a hardened layer on the outer surface of the skirt is abraded more than the other part of the hardened layer, so that the hardened layer will not be uniform in thickness throughout the piston. As a result, desired abrasion resistance cannot be obtained. This is a problem to be solved and it may occur particularly with regard to a piston of ball joint type, which comprises a skirt having a relatively long thin portion.
In view of the above, the present invention provides a method for manufacturing a piston which has a hardened layer uniform in thickness formed on the whole outer surface without weight increase to highly resists abrasion.